1. Field of the Invention
The present invention concerns a method to operate an imaging system, in particular a magnetic resonance system, with a number of subsystems and a control device that controls the subsystems in a coordinated manner to implement a measurement sequence. For this sequence control data which define different functional sub-sequences of the measurement sequence are provided to the control device, wherein different active volumes are associated with the functional sub-sequences. Moreover, the invention concerns an imaging system with a number of subsystems and a control device to implement this method.
2. Description of the Prior Art
Tomographical imaging systems (for example magnetic resonance apparatuses or computed tomography systems) are complex installations with multiple technical subsystems. Among these (for example in a magnetic resonance system) are the basic field magnet system in order to expose a body to be examined to a relatively high basic magnetic field, for example 1.5 Tesla or even 3 Tesla in newer systems, known as high magnetic field systems; a gradient system in order to additionally apply a magnetic field gradient; and a shim system in order to homogenize the magnetic fields. Moreover, such a magnetic resonance system has a radio-frequency transmission system in order to emit a radio-frequency excitation signal with suitable antenna devices. This excitation signal causes the nuclear spins of specific atoms excited to resonance by this radio-frequency field to be tilted by a specific flip angle relative to the magnetic field lines of the basic magnetic field. An additional subsystem required by the magnetic resonance system is a radio-frequency reception system which serves to receive and additionally process the radio-frequency signal radiated upon relaxation of the nuclear spins (known as the magnetic resonance signal) so that the desired image data can be reconstructed from the raw data acquired in this manner. For spatial coding, defined magnetic field gradients respectively generated by means of a gradient system are superimposed on the basic magnetic field during the transmission and readout or (reception) of the radio-frequency signals.
All of these technical modules must be operated in a coordinated fashion and in a suitable manner by a controller. The adjustments and switchings of the individual subsystems that are necessary for a specific imaging process must be activated at the respective correct points in time. Within an imaging workflow, the volume to be imaged is typically acquired in sub-volumes, for example in multiple slices in 2D imaging or in what are known as multiple “slabs” in 3D imaging. The sub-volumes that are acquired in this way are then assembled into a complete volume. An additional definition of sub-volumes can result via “regions of interest” that can be specifically defined by the operator, for example. Furthermore, additional sub-volumes result given the establishment of local saturation regions or local preparation or labeling pulses, for example in magnetic resonance systems.
As mentioned above, sequence control data (most often within a measurement protocol) are transmitted to the control device for coordinated control. These sequence control data define different functional sub-sequences of a complete measurement sequence. For example, in a magnetic resonance acquisition a first sub-sequence can be a pulse sequence in order to locally achieve a saturation in a specific region. Additional sub-sequences can contain specific preparation pulses, for example, and other sub-sequences serve again for the successive excitation and to receive the magnetic resonance signals in different slices or slabs. It is normally the case that different active volumes are associated with the different functional sub-sequences, meaning that a different sub-volume of the entire measurement volume is relevant for each sub-sequence. In general, however, no information or at best limited information about the occupation in space (i.e. the position and orientation) and the extent of the different sub-volumes is provided to the technical subsystems. Therefore, information about the spatial occupation and extent of the sub-volumes have previously been used only in the spatial selection (i.e. given a specific slice and slab excitation or a very specific regional saturation) in the control of the individual subsystems in tomographical imaging methods. For example, in magnetic resonance apparatuses a slice-selection gradient is applied in a targeted manner simultaneously with a radio-frequency excitation pulse of suitable shape and frequency in order to excite a specific slice. Moreover, it has previously been the case that only proprietary optimization methods have been known in which it is sought to optimize the image quality via specific activation of individual subsystems depending on defined sub-volumes.
In DE 10 2004 002 009 A1 a method is described for local homogenization of the radio-frequency field distribution of RF pulses in a determinable active volume. In U.S. Pat. No. 7,372,270 a method is likewise described for compensation of inhomogeneities of the RF excitation field. A method to measure the RF field distribution for a possible optimization is described in DE 103 38 075 A1 In U.S. Pat. No. 6,509,735 a method is explained for updating the global imaging parameter given a movement of an examination subject. DE 102 14 736 A1 is concerned with the optimization of k-space trajectories, i.e. with the optimal adjustments of the gradients for spatial coding in a magnetic resonance apparatus. However, all of these methods deal only with the setting of individual subsystems of the magnetic resonance system.
However, in most systems the actual optimization of the different sub-sequences to the associated active volumes ensues only by virtue of the developer of a control protocol modifying the sequence control data in a suitable manner, meaning that he or she must calculate, in a suitably tailored sequence, control data on the basis of his or her knowledge of the desired sub-volumes, or the active volumes belonging to the sub-sequences, and then correspondingly change the control protocol so that a suitable control locally optimized to the active volume ensues in this way for the individual sub-sequences.
This method is extraordinarily time-consuming, and places markedly high demands on the developers of the control sequences. A “normal” operator of a magnetic resonance apparatus is then generally no longer in the position to vary a control protocol (if necessary) and adapt it to an examination without having to be concerned that the optimization of the individual sub-sequences with regard to the associated active volumes will be lost. Furthermore, the developer of the control sequence in this case must have detailed knowledge about the subsystems in order to be able to make the corresponding modifications at all. A poorer optimization in turn leads to worse measurement results, which in the extreme case can lead to the situation that the generated images are not useful and a greater risk of misinterpretations exists, or measurements must be repeated, which causes additional stress for the patient.